In some instances the males alone, in
other instances, both males and females, have been observed thus to differ
in a slight degree. When the differences are rather more strongly marked,
and when both sexes and all ages are affected, the forms are ranked by all
entomologists as good species. But no observer can determine for another,
even if he can do so for himself, which of these Phytophagic forms ought to
be called species and which varieties. Mr. Walsh ranks the forms which it
may be supposed would freely intercross, as varieties; and those which
appear to have lost this power, as species. As the differences depend on
the insects having long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found. The
naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or
islands. When, on the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipelago, and presents
different forms in the different areas, there is always a good chance that
intermediate forms will be discovered which will link together the extreme
states; and these are then degraded to the rank of varieties.
Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific
value; and when the same identical form is met with in two distant
countries, or in two geological formations, they believe that two distinct
species are hidden under the same dress.
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